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Located just 36 miles north of San Diego, Carlsbad, Calif., has become an international tourist destination. With attractions such as the world famous Flower Fields at Carlsbad Ranch and LegoLand, the city is quickly growing and in need of more retail shops and restaurants. In recent years, most land in the coastal area of the city has been developed.
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One of the last pieces of prime commercial real estate is situated on Paseo Del Norte, east of I-5 between Cannon Road and Palomar Airport Road. This 7-acre plot of land is a perfect spot for a California Spanish architecture-inspired complex. It will include high-end retailers, homey boutiques, salon services, and popular restaurants.
However, complying with stormwater regulations for coastal commercial developments has become increasingly difficult. New regulations require low-impact development (LID) strategies that can take up significant amounts of space, thus limiting the amount of buildable land. With the high cost of prime coastal land, implementing traditional LID strategies would make the project unfeasible.
Project Engineer Gary Lipska of Aquaterra Engineering Inc. contacted Bio Clean Environmental Services Stormwater Engineer Zach Kent in the fall of 2008 in search of a solution. The project was required to meet standard mitigation of pollutants generated by the site, along with LID and hydromodification requirements.
The challenge
The project’s hydromodification requirements meant that any increase in runoff volume had to be infiltrated or detained onsite. Storage facilities would need to infiltrate and detain the flow of a 10-year storm event, which generates a large quantity of water; finding a place to store it would be essential.
Providing ample parking spots for this size project was another main concern. Storing water in an open catch basin would take up substantial buildable space, thus limiting the number of spots. More than 500 parking spots would be needed to accommodate the traffic. Another challenge was dealing with the pollutants generated by the parking lot and restaurants. The pollutants of concern for these land uses are: trash and debris, oil and grease, sediments, organics, and heavy metals from vehicles such as antifreeze, brake fluid, fuel, and transmission fluid.
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“The Carlsbad Paseo project had several challenging design drainage features. Not only was retention and treatment of the onsite drainage required, but the pass-through drainage from the upstream commercial properties had to be treated,” Lipska explained. This also was a high-profile project that required 401 approvals by the San Diego Regional Water Quality Control Board because it discharged to a sensitive water body on the 303 (d) list.
The solution
After only a week in conceptual design, the stormwater engineering staff at Bio Clean drafted a proposal and concept that they believed the city of Carlsbad and San Diego Regional Water Quality Control Board would approve. Because of increased emphasis on using treatment trains, Bio Clean proposed a series of three structural best management practices (BMPs).
A total of 21 Grate Inlet Skimmer Boxes (GISBs), six Nutrients Separating Baffle Boxes (NSBBs), and six separate StormTank underground infiltration/detention systems were used to establish a treatment train system to filter stormwater runoff before infiltrating back into the natural soils or discharging downstream. The entire treatment process and detention system is located beneath the parking lot. This solution maximized buildable space of the project.
The first stage of treatment is through a GISB, which is installed at each of the 21 grate openings. The GISB is constructed of all-fiberglass frame and stainless steel. It is fitted with an absorbent media for removal of petroleum hydrocarbons. These filters have an expected service life of 10 to 15 years. They are designed to allow for a high-flow bypass, which will not inhibit storm flows and cause flooding while capturing high levels of trash and litter, grass and foliage, sediments, hydrocarbons, grease, and oil.
The second stage of treatment, the NSBBs, are located downstream of the inlets. Upon entering the system, the flows travel through a patented filtration screen system that captures and stores trash and organics in a dry state above the standing water, which prevents septic conditions from occurring. Sediments and fine total suspended solids drop into one of the three settling chambers, maximizing capture. Before leaving the system, runoff travels through a skimmer and box stage that collects hydrocarbons and controls flow velocity, which improves removal efficiency.
The last stage of the treatment process involves the flow entering into a StormTank underground detention/infiltration system. This system allows large volumes of runoff to be contained underground and then infiltrated into the native soil or discharged downstream in a controlled fashion. The StormTank offers 97 percent void space, therefore maximizing storage capacity. The initial chamber, 6 feet by 6 feet, is double-wrapped with geotextile fabric to ensure sediments do not travel further into the system. Above this initial chamber is a manhole to allow for easy maintenance with a vacuum truck to clean debris and sediment.
The result
The city of Carlsbad approved the development in fall 2008, and construction of the two restaurants, P.F. Chang’s China Bistro and BJ’s Restaurant & Brewhouse, began in March 2009. In addition to these, a two-story structure consisting of 45,000 square feet will be built between the restaurants. This structure will host retailers, salons, and a place to relax.
Before this system was implemented, the runoff emptied into Carlsbad’s Agua Hedionda Lagoon just north of Cannon Road.
“Using this type of treatment train has proved to be the best type of defense against stormwater runoff,” said Bio Clean’s Kent. “Eliminating and reducing runoff from this site will benefit the surrounding bodies of water.”
Abigail Kent is director of marketing for Bio Clean Environmental Services Inc. She can be contacted at abby@biocleanenvironmental.net. Gary Lipska, Aquaterra Engineering Inc., also contributed to this article.
